We investigated the structure of the 107-kD thyroid protein recognized as microsomal antigen. Solubilized microsomes were incubated with affinity gels consisting of IgG, from thyroiditis patients or controls, linked to Reacti-gel. Eluates were analyzed by SDS polyacrylamide electrophoresis and Western blot. 107- and 101-kD proteins were augmented in eluates from gels containing patient IgG and had microsomal antigenicity. In a Western blot of microsomes run under unreduced conditions, poorly defined large proteins were identified by antibody. When eluted electrophoretically and reanalyzed in reducing conditions, they demonstrated the 107-kD antigen. The 107-kD protein identified in reducing conditions was extracted and reanalyzed under nonreducing conditions. Large molecular mass proteins were then observed. On two-dimensional electrophoresis, a 107-kD antigen was isolated with isoelectric point of 7.0. The microsomal antigen may be complexes or multimers of a 107-kD peptide with isoelectric point of 7.0.